1.
Sunset
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Sunset or sundown, is the daily disappearance of the Sun below the horizon as a result of Earths rotation. The Sun will set exactly due west at the equator on the spring and fall equinoxes, the time of sunset is defined in astronomy as the moment when the trailing edge of the Suns disk disappears below the horizon. Dusk is at the end of astronomical twilight, and is the darkest moment of twilight just before night. Sunset creates unique atmospheric conditions such as the intense orange and red colors of the Sun. The time of sunset varies throughout the year, and is determined by the position on Earth, specified by longitude and latitude. During winter and spring, the days get longer and sunsets occur later every day until the day of the latest sunset, in the Northern Hemisphere, the latest sunset occurs late in June or in early July, but not on the summer solstice of June 21. This date depends on the viewers latitude, likewise, the earliest sunset does not occur on the winter solstice, but rather about two weeks earlier, again depending on the viewers latitude. In the Northern Hemisphere, it occurs in early December or late November, for a few weeks surrounding both solstices, both sunrise and sunset get slightly later each day. Even on the equator, sunrise and sunset shift several minutes back and forth through the year and these effects are plotted by an analemma. Sunsets occur almost exactly due west on the equinoxes for all viewers on Earth, exact calculations of the azimuths of sunset on other dates are complex, but they can be estimated with reasonable accuracy by using the analemma. As sunrise and sunset are calculated from the leading and trailing edges of the Sun, respectively, and not the center, the duration of a daytime is slightly longer than nighttime. Further, because the light from the Sun is refracted as it passes through the Earths atmosphere, refraction also affects the apparent shape of the Sun when it is very close to the horizon. It makes things appear higher in the sky than they really are, light from the bottom edge of the Suns disk is refracted more than light from the top, since refraction increases as the angle of elevation decreases. This raises the apparent position of the edge more than the top. Its width is unaltered, so the disk appears wider than it is high, the Sun also appears larger on the horizon, an optical illusion, similar to the moon illusion. Because the shorter wavelength components, such as blue and green, scatter more strongly, the remaining reddened sunlight can then be scattered by cloud droplets and other relatively large particles to light up the horizon red and orange. The removal of the wavelengths of light is due to Rayleigh scattering by air molecules and particles much smaller than the wavelength of visible light. The scattering by cloud droplets and other particles with diameters comparable to or larger than the sunlights wavelengths is due to Mie scattering and is not strongly wavelength-dependent
2.
Sunrise
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Sunrise or sun up is the instant at which the upper edge of the Sun appears over the horizon in the morning. The term can refer to the entire process of the Sun crossing the horizon. Although the Sun appears to rise from the horizon, it is actually the Earths motion that causes the Sun to appear, architect Buckminster Fuller proposed the terms sunsight and sunclipse to better represent the heliocentric model, though the terms have not entered into common language. Astronomically, sunrise occurs for only an instant, the moment at which the limb of the Sun appears tangent to the horizon. However, the term commonly refers to periods of time both before and after this point, Twilight, the period in the morning during which the sky is light. The beginning of morning twilight is called dawn, the period after the Sun rises during which striking colors and atmospheric effects are still seen. Sunrise occurs before the Sun actually reaches the horizon because the Suns image is refracted by the Earths atmosphere, at the horizon, the average amount of refraction is 34 arcminutes, though this amount varies based on atmospheric conditions. Also, unlike most other measurements, sunrise occurs when the Suns upper limb, rather than its center. The apparent radius of the Sun at the horizon is 16 arcminutes and these two angles combine to define sunrise to occur when the Suns center is 50 arcminutes below the horizon, or 90. 83° from the zenith. The timing of sunrise varies throughout the year and is affected by the viewers longitude and latitude, altitude. The analemma can be used to make predictions of the time of sunrise. In late winter and spring, sunrise as seen from temperate latitudes occurs earlier each day, reaching its earliest time near the summer solstice, after this point, the time of sunrise gets later each day, reaching its latest sometime around the winter solstice. Variations in atmospheric refraction can alter the time of sunrise by changing its apparent position, near the poles, the time-of-day variation is exaggerated, since the Sun crosses the horizon at a very shallow angle and thus rises more slowly. Accounting for atmospheric refraction and measuring from the leading edge slightly increases the duration of day relative to night. Sunrises occur approximately due east on the March and September equinoxes for all viewers on Earth, exact calculations of the azimuths of sunrise on other dates are complex, but they can be estimated with reasonable accuracy by using the analemma. Air molecules and airborne particles scatter white sunlight as it passes through the Earths atmosphere and this is done by a combination of Rayleigh scattering and Mie scattering. Because the shorter wavelength components, such as blue and green, scatter more strongly, the remaining reddened sunlight can then be scattered by cloud droplets and other relatively large particles to light up the horizon red and orange. The removal of the wavelengths of light is due to Rayleigh scattering by air molecules and particles much smaller than the wavelength of visible light
3.
Season
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A season is a division of the year marked by changes in weather, ecology and hours of daylight. Seasons result from the orbit of the Earth around the Sun. During May, June, and July, the northern hemisphere is exposed to direct sunlight because the hemisphere faces the sun. The same is true of the hemisphere in November, December. It is the tilt of the Earth that causes the Sun to be higher in the sky during the months which increases the solar flux. However, due to lag, June, July, and August are the hottest months in the northern hemisphere and December, January. In temperate and subpolar regions, four calendar-based seasons are recognized, spring, summer, autumn or fall. Ecologists often use a model for temperate climate regions, prevernal, vernal, estival, serotinal, autumnal. Many tropical regions have two seasons, the rainy, wet, or monsoon season and the dry season, some have a third cool, mild, or harmattan season. Seasons often held special significance for agrarian societies, whose lives revolved around planting and harvest times, in some parts of the world, some other seasons capture the timing of important ecological events such as hurricane season, tornado season, and wildfire season. The most historically important of these are the three seasons—flood, growth, and low water—which were previously defined by the annual flooding of the Nile in Egypt. The seasons result from the Earths axis of rotation being tilted with respect to its orbital plane by an angle of approximately 23.5 degrees, regardless of the time of year, the northern and southern hemispheres always experience opposite seasons. This is because during summer or winter, one part of the planet is directly exposed to the rays of the Sun than the other. For approximately half of the year, the northern hemisphere tips toward the Sun, for the other half of the year, the same happens, but in the southern hemisphere instead of the northern, with the maximum around December 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes. Also at that moment, both the North Pole and the South Pole of the Earth are just on the terminator, and hence day and night are equally divided between the northern and southern hemispheres. Around the March equinox, the northern hemisphere will be experiencing spring as the hours of daylight increase, the effect of axial tilt is observable as the change in day length and altitude of the Sun at noon during a year. Between this effect and the daylight hours, the axial tilt of the Earth accounts for most of the seasonal variation in climate in both hemispheres
4.
Night
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Night or nighttime is the period of time between the sunset and the sunrise when the Sun is below the horizon. Complete darkness or astronomical night is the period of time between astronomical dusk and astronomical dawn when the sun is between 18 and 90 degrees below the horizon and the sky is totally dark, at latitudes between 48.5 and 66. The opposite of night is day, the start and end points of time for a night vary, based on factors such as season, latitude, longitude and timezone. Twilight is the time of night when the sky is illuminated by the sun but the sun is below the horizon. At any given time, one side of the planet Earth is bathed in light from the Sun and this shadow is called the umbra. Natural illumination is provided by a combination of moonlight, planetary light, starlight, diffuse zodiacal light, gegenschein. In some circumstances, bioluminescence, aurorae, and lightning can provide some illumination, the glow provided by artificial illumination is sometimes referred to as light pollution because it can interfere with observational astronomy and ecosystems. Nights are shorter than days on average due to two factors, firstly, the sun is not a point, but has an apparent size of about 32 arc minutes. Secondly, the atmosphere refracts sunlight so that some of it reaches the ground when the sun is below the horizon by about 34 arc minutes. The combination of two factors means that light reaches the ground when the center of the sun is below the horizon by about 50 arc minutes. Without these effects, day and night would be the length at the autumnal and vernal equinoxes. In reality, around the equinoxes the day is almost 14 minutes longer than the night at the equator, the summer and winter solstices mark the shortest and the longest night, respectively. The closer a location is to either the North Pole or the South Pole, in the Northern Hemisphere, Denmark has shorter nights in June than India. In the Southern Hemisphere, Antarctica has longer nights in June than Chile, the phenomenon of day and night is due to the rotation of a celestial body about its axis, creating an illusion of the sun rising and setting. Different bodies spin at different rates, however. Some may spin much faster than Earth, while others spin extremely slowly, leading to long days. The planet Venus rotates once every 224.7 days – by far the slowest rotation period of any of the major planets, in contrast, the gas giant Jupiters sidereal day is only 9 hours and 56 minutes. Mercury has the longest day-night cycle as a result of its 3,2 resonance between its orbital period and rotation period - this resonance gives it a day-night cycle 176 days long
5.
Dusk
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Dusk occurs at the darkest stage of twilight, or at the very end of astronomical twilight after sunset and just before night. The term dusk usually refers to astronomical dusk, or the darkest part of twilight before night begins. The time of dusk is the moment at the end of astronomical twilight, just before total darkness or night. Civil twilight, the first phase of twilight, ends when the sun is 6 degrees below the horizon in the evening, at this time objects are still distinguishable and depending on weather conditions some stars and planets may start to become visible to the naked eye. Nautical twilight, the phase of twilight, ends when the sun is 12 degrees below the horizon in the evening. At this time, objects are less distinguishable, and stars, astronomical twilight, the final phase of twilight, ends when the sun is 18 degrees below the horizon in the evening. At this time the sun no longer illuminates the sky, and thus no longer interferes with astronomical observations
6.
Dawn
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Dawn or astronomical dawn is the time that marks, depending on the specific usage, the beginning of the twilight before sunrise, the period of the pre-sunrise twilight or the time of sunrise. When identified as the beginning of or the period of twilight, it is recognized by the presence of weak sunlight, different definitions exist for the start of dawn. The difference between these definitions is the amount of sunlight that must be present and this occurs when the Sun is 18 degrees below the horizon in the morning. Though it is possible to localize the direction of the Sun during astronomical dawn and dusk, astronomical dawn and dusk are night, the zenith is extremely dark and more than just the brightest stars can be seen. Nautical Dawn is still dark and is considered night. At civil dawn there is darkness and at the zenith. The sky is still a bit dark and civil dawn is still considered night, at all places on the earth, dawn and dusk times are fastest around the equinoxes and slowest at the summer and winter solstices. As the calendar approaches the summer or winter solstice, the days or nights, respectively, get longer, which can have a potential impact on the time and duration of dawn and dusk. This effect is pronounced closer to the poles, where the Sun rises at the spring equinox and sets at the autumn equinox, with a long period of dawn/dusk. The polar circle is defined as the lowest latitude at which the Sun does not set at the summer solstice, therefore, the angular radius of the polar circle is equal to the angle between the plane of Earths equator and that of the ecliptic. This period of time with no sunset lengthens closer to the pole, near the summer solstice, latitudes higher than 54°30′ get no darker than nautical dawn/dusk, the darkness of the night varies greatly in these latitudes. At latitudes higher than about 59°20, summer nights get no darker than civil dusk or dawn and this period of bright nights is longer at higher latitudes. When the sun gets 9.0 to 9.5 degrees below the horizon, many Indo-European mythologies have a dawn goddess, separate from the male Solar deity, her name deriving from PIE *h2ausos-, derivations of which include Greek Eos, Roman Aurora and Indian Ushas. Also related is Lithuanian Aušrinė, and possibly a Germanic *Austrōn-, in Sioux mythology, Anpao is an entity with two faces. The Hindu dawn deity Ushas is female, whereas Surya, the Sun, and Aruṇa, Ushas is one of the most prominent Rigvedic deities. The time of dawn is also referred to as the Brahmamuhurtham, in some parts of India, both Usha and Pratyusha are worshiped along with the Sun during the festival of Chhath. Prime is the time of prayer of the traditional Divine Office in Christian liturgy. In Islam, astronomical dawn is the time of the first prayer of the day, in Judaism, the question of how to calculate dawn is posed by the Talmud, as it is has many ramifications for Jewish law
7.
24-hour clock
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The 24-hour clock is the convention of time keeping in which the day runs from midnight to midnight and is divided into 24 hours, indicated by the hours passed since midnight, from 0 to 23. This system is the most commonly used time notation in the world today, in the practice of medicine, the 24-hour clock is generally used in documentation of care as it prevents any ambiguity as to when events occurred in a patients medical history. It is popularly referred to as time in the United States. In the case of a second, the value of ss may extend to 60. A leading zero is added for numbers under 10 and this zero is optional for the hours, but very commonly used in computer applications, where many specifications require it. Where subsecond resolution is required, the seconds can be a fraction, that is. The most commonly used separator symbol between hours, minutes and seconds is the colon, which is also the used in ISO8601. In the past, some European countries used the dot on the line as a separator, in some contexts, no separator is used and times are written as, for example,2359. In East Asia, time notation was 24-hour before westernization in modern times, the clocks were changed into 12 dual-hours style when they were shipped to China in the Qing dynasty. In the 24-hour time notation, the day begins at midnight,00,00, and the last minute of the day begins at 23,59. Where convenient, the notation 24,00 may also be used to refer to midnight at the end of a given date – that is,24,00 of one day is the time as 00,00 of the following day. The notation 24,00 mainly serves to refer to the end of a day in a time interval. A typical usage is giving opening hours ending at midnight, similarly, some railway timetables show 00,00 as departure time and 24,00 as arrival time. Legal contracts often run from the date at 00,00 until the end date at 24,00. While the 24-hour notation unambiguously distinguishes between midnight at the start and end of any date, there is no commonly accepted distinction among users of the 12-hour notation. Sometimes the use of 00,00 is also avoided, in variance with this, the correspondence manual for the U. S. Navy and U. S. Marine Corps formerly specified 0001 to 2400. The manual was updated in June 2015 to use 0000 to 2359, time-of-day notations beyond 24,00 are not commonly used and not covered by the relevant standards. In most countries, computers by default show the time in 24-hour notation, for example, Microsoft Windows and macOS activate the 12-hour notation by default only if a computer is in a handful of specific language and region settings
8.
Hebrew calendar
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The Hebrew or Jewish calendar is a lunisolar calendar used today predominantly for Jewish religious observances. It determines the dates for Jewish holidays and the public reading of Torah portions, yahrzeits. The present Hebrew calendar is the product of evolution, including a Babylonian influence, when to add it was based on observation of natural agriculture-related events in Israel. Through the Amoraic period and into the Geonic period, this system was displaced by the mathematical rules used today. The principles and rules were codified by Maimonides in the Mishneh Torah in the 12th century. Maimonides work also replaced counting years since the destruction of the Temple with the modern creation-era Anno Mundi, the era used since the Middle Ages is the Anno Mundi epoch. As with Anno Domini, the words or abbreviation for Anno Mundi for the era should properly precede the date rather than follow it, AM5777 began at sunset on 2 October 2016 and will end at sunset on 20 September 2017. The Jewish day is of no fixed length, the Jewish day is modeled on the reference to. there was evening and there was morning. In the creation account in the first chapter of Genesis, based on the classic rabbinic interpretation of this text, a day in the rabbinic Hebrew calendar runs from sunset to the next sunset. In most populated parts of the world this is always approximately 24 standard hours, halachically, a day ends and a new one starts when three stars are visible in the sky. The time between sunset and the time when the three stars are visible is known as bein hashmashot, and there are differences of opinion as to which day it falls into for some uses. This may be relevant, for example, in determining the date of birth of a child born during that gap, there is no clock in the Jewish scheme, so that the local civil clock is used. The civil clock is used only as a reference point – in expressions such as, Shabbat starts at. In Judaism, an hour is defined as 1/12 of the time from sunrise to sunset, so, during the winter, an hour can be less than 60 minutes. This proportional hour is known as a shaah zmanit, a Jewish hour is divided into 1080 halakim or parts. A part is 3⅓ seconds or 1/18 minute, the ultimate ancestor of the helek was a small Babylonian time period called a barleycorn, itself equal to 1/72 of a Babylonian time degree. These measures are not generally used for everyday purposes, instead of the international date line convention, there are varying opinions as to where the day changes. One opinion uses the antimeridian of Jerusalem, the weekdays start with Sunday and proceed to Saturday, Shabbat
9.
New Year's Eve
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In the Gregorian calendar, New Years Eve, the last day of the year, is on December 31 which is the seventh day of the Christmas season. In many countries, New Years Eve is celebrated at evening gatherings, where many people dance, eat, drink alcoholic beverages. Some Christians attend a watchnight service, the celebrations generally go on past midnight into January 1. Samoa, Tonga and Kiritimati, part of Kiribati, are the first places to welcome the New Year while American Samoa, New Year traditions and celebrations in Canada vary regionally. New Years Eve is generally a social holiday, in some areas, such as in rural Quebec, people ice fish in the old days. Since 2000, the highlight of New Years Eve celebrations is in Montreals old port, which comes alive with concerts that take place and fireworks at midnight. From 1956 to 1976, Guy Lombardo and his Royal Canadians serenaded Canada on the CBC, via a feed from CBS, after Lombardos death in 1977, the Royal Canadians continued on CBC and CBS until 1978. Following the finale of the series, the original cast continued to participate in New Years Eve specials in the years following. Similarly, the CBCs French language network Ici Radio-Canada Télé airs its own yearly New Years Eve comedy special, Bye Bye. Unlike Year of the Farce, Bye Bye has been presented by comedians, originally running from 1968 to 1998. Mexicans celebrate New Years Eve, by eating a grape with each of the chimes of a clocks bell during the midnight countdown. Mexican sweet bread is baked with a coin or charm hidden in the dough, when the bread is served, the recipient of the slice with the coin or charm is said to be blessed with good luck in the New Year. At the same time, they are expressed for all the good things during the year that is ending so that they continue in the new year. Mexicans celebrate with a dinner with their families, the traditional meal being turkey or pork loin. Afterwards many people attend parties outside the home, for example, in Mexico City there is a street festival on New Years Eve centered on the Zocalo, the citys main square. Celebrations include firecrackers, fireworks and sparklers and shouts of ¡Feliz Año Nuevo, in Puerto Rico, New Years Eve is celebrated with friends and family. The Puerto Rico Convention Center in San Juan is the attraction for Puerto Ricans during the celebration. It has Latin music and fireworks at midnight along with the signature song Auld Lang Syne in Spanish, in the United States, New Years Eve is celebrated with formal parties, family-oriented activities, and other large public events
10.
National Institute of Standards and Technology
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The National Institute of Standards and Technology is a measurement standards laboratory, and a non-regulatory agency of the United States Department of Commerce. Its mission is to promote innovation and industrial competitiveness, in 1821, John Quincy Adams had declared Weights and measures may be ranked among the necessities of life to every individual of human society. From 1830 until 1901, the role of overseeing weights and measures was carried out by the Office of Standard Weights and Measures, president Theodore Roosevelt appointed Samuel W. Stratton as the first director. The budget for the first year of operation was $40,000, a laboratory site was constructed in Washington, DC, and instruments were acquired from the national physical laboratories of Europe. In addition to weights and measures, the Bureau developed instruments for electrical units, in 1905 a meeting was called that would be the first National Conference on Weights and Measures. Quality standards were developed for products including some types of clothing, automobile brake systems and headlamps, antifreeze, during World War I, the Bureau worked on multiple problems related to war production, even operating its own facility to produce optical glass when European supplies were cut off. Between the wars, Harry Diamond of the Bureau developed a blind approach radio aircraft landing system, in 1948, financed by the Air Force, the Bureau began design and construction of SEAC, the Standards Eastern Automatic Computer. The computer went into operation in May 1950 using a combination of vacuum tubes, about the same time the Standards Western Automatic Computer, was built at the Los Angeles office of the NBS and used for research there. A mobile version, DYSEAC, was built for the Signal Corps in 1954, due to a changing mission, the National Bureau of Standards became the National Institute of Standards and Technology in 1988. Following 9/11, NIST conducted the investigation into the collapse of the World Trade Center buildings. NIST had a budget for fiscal year 2007 of about $843.3 million. NISTs 2009 budget was $992 million, and it also received $610 million as part of the American Recovery, NIST employs about 2,900 scientists, engineers, technicians, and support and administrative personnel. About 1,800 NIST associates complement the staff, in addition, NIST partners with 1,400 manufacturing specialists and staff at nearly 350 affiliated centers around the country. NIST publishes the Handbook 44 that provides the Specifications, tolerances, the Congress of 1866 made use of the metric system in commerce a legally protected activity through the passage of Metric Act of 1866. NIST is headquartered in Gaithersburg, Maryland, and operates a facility in Boulder, nISTs activities are organized into laboratory programs and extramural programs. Effective October 1,2010, NIST was realigned by reducing the number of NIST laboratory units from ten to six, nISTs Boulder laboratories are best known for NIST‑F1, which houses an atomic clock. NIST‑F1 serves as the source of the official time. NIST also operates a neutron science user facility, the NIST Center for Neutron Research, the NCNR provides scientists access to a variety of neutron scattering instruments, which they use in many research fields
